1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling uplink timing and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple UEs, and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (COMP), uplink (UL) multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
A hybrid automatic repeat request (HARQ) process is used in a communication system (e.g., the LTE system and the LTE-A system) to provide both efficient and reliable communications. Different from an automatic repeat request (ARQ) process, a forward error correcting code (FEC) and soft combining are used for the HARQ process. In detail, before a transmitter (e.g., eNB) transmits a packet (e.g., a data stream, a frame or a transport block) including multiple coded bits to a receiver (e.g., UE), the transmitter divides the packet into multiple blocks, i.e., multiple redundancy versions. The transmitter only transmits one of the redundancy versions in each transmission or retransmission. According to whether the same redundancy version is transmitted in the retransmission, the soft combining used for the HARQ can be classified into two categories: chase combining (CC) and incremental redundancy (IR). When the same redundancy version of the packet is transmitted in each retransmission, the HARQ is a CC-based HARQ. When a different redundancy version of the packet is transmitted in each retransmission, the HARQ is an IR-based HARQ.
The CA is introduced to the LTE-A system according to which two or more component carriers are aggregated to achieve a wider-band transmission. Accordingly, the LTE-A system can support a wider bandwidth up to 100 MHz by aggregating a maximum number of 5 component carriers, where bandwidth of each component carrier is 20 MHz and is backward compatible with 3GPP Rel-8. The LTE-A system supports the CA for both continuous and non-continuous component carriers. The CA increases bandwidth flexibility by aggregating the non-continuous component carriers.
When the UE is configured with the CA, the UE is allowed to receive and transmit data on one or multiple component carriers to increase the data rate. In the LTE-A system, it is possible for the eNB to configure the UE different numbers of UL and DL component carriers which depend on UL and DL aggregation capabilities, respectively. Moreover, the component carriers configured to the UE necessarily consist of one DL primary component carrier (PCC) and one UL primary component carrier. Component carriers other than the primary component carriers are UL or DL secondary component carriers (SCCs). The numbers of the UL and DL secondary component carriers are arbitrary, and are related to the UE capability and available radio resources. Further, a cell operating on the primary component carrier is termed a primary cell (PCell), and a cell operating on the secondary component carrier is termed a secondary cell (SCell). When the CA is configured, the UE only has one radio resource control (RRC) connection with the network. At establishment of the RRC connection, re-establishment of the RRC connection or a handover, the PCell provides the NAS mobility information. Further, at the re-establishment of the RRC connection or the handover, the PCell provides the security input. Depending on UE capabilities, one or more SCells can be configured together with the PCell to form a set of serving cells for the UE. The reconfiguration, addition and removal of an SCell can be performed via the RRC.
To enable reasonable UE battery consumption when the CA is configured, an activation/deactivation mechanism of the SCells is supported. For example, an SCell added to a set of serving cells of a UE via the RRC is initially deactivated. When the SCell is deactivated, the UE does not need to receive a PDCCH or a PDSCH corresponding to the SCell, cannot transmit in the corresponding uplink, nor is it required to perform channel quality indicator (CQI) measurements. Conversely, when an SCell is activated, the UE shall receive a PDSCH and a PDCCH corresponding to the SCell (if the UE is configured to monitor the PDCCH from this SCell), and is expected to be able to perform the CQI measurements. The activation/deactivation mechanism is operated according to the combination of a medium access control (MAC) control element and deactivation timers. The MAC control element carries a bitmap for the activation and deactivation of SCells: a bit set to 1 denotes activation of the corresponding SCell, while a bit set to 0 denotes deactivation. With the bitmap, SCells can be activated and deactivated individually, and a single activation/deactivation command can activate/deactivate a subset of the SCells. A deactivation timer is maintained per SCell but one common value is configured per UE by RRC. When a deactivation timer associated with a SCell expires, the SCell is deactivated.
On the other hand, when a UE is configured with a cell, the UE needs to maintain a UL timing of the cell for UL transmission, to communicate with the cell synchronously. In short, before a time alignment timer expires, the UE needs to monitor a DL signaling of the cell, to know a DL timing. Then, the UE can adjust the UL timing for a UL frame transmission according to the DL timing. The DL timing is defined as a time when the first detected path (in time) of a corresponding DL frame is received from the cell. The UE adjusts the UL timing by transmitting the UL frame at a time before the DL timing of the corresponding DL frame. Details about how to adjust the UL timing are specified in the 3GPP Technical Specification 36.133 v10.3.0. In the LTE-A system supporting the 3GPP Rel-10 standard, UL timings of cells (including the primary cell and one or more secondary cells) configured to the UE are the same, and the UE can easily maintain the UL timing. That is, there is only one timing advance (TA) group, wherein the UL timings of the cells in the TA group are the same. The UE adjusts the UL timing for the UL transmission according to a DL timing of the PCell. However, deployment of eNBs is restricted due to that the UL timings must be same. Thus, different UL timings are allowed for different cells in the LTE-A system support later versions of the 3GPP standard, to release the restriction. That is, multiple TA groups are possible. Thus, there is still one UL timing for cells of a TA group, while UL timings of different TA groups can be different. In a TA group a cell whose DL timing is used by the UE for adjusting a UL timing of cells in the TA group is called a timing reference cell of the TA group. Cells in a TA group have UL to which the same timing advance applies.
However, it is not known how to maintain one or more UL timings efficiently (e.g., with low power consumption), when multiple UL timings are possible. Besides, when a time alignment timer of a TA group expires, the UE clears (i.e., flushes) all HARQ buffers of the UE. It is not known whether HARQ buffers corresponding to cells in other TA groups should also be cleared. Thus, how to solve the abovementioned problem is a topic to be discussed.